1. Field of the Invention
This invention relates to polysiloxanes containing both alkylhydrosiloxane units and alkylvinylsiloxane units which polysiloxanes are siloxy-endblocked. These polysiloxanes are useful as starting materials for silicon oxycarbide ceramic compositions. This invention also relates to a process for the preparation of such polysiloxanes, to a process for the conversion of such polysiloxanes to ceramic compositions, and to the ceramic compositions so prepared.
2. Description of the Prior Art
Polysiloxanes containing methylhydrosiloxane and methylvinylsiloxane units have been prepared as by-products in the preparation of cyclic siloxanes by hydrolysis of chlorosilanes (K. A. Andrianov, et al., Zhur. Obshch. Khim., 35, 103 (1965); 35 524 (1965); and J. Organometal Chem., 4, 440 (1965)). The polysiloxanes of the above references containing these units were hydroxy-endblocked, i.e. terminated by hydroxyl groups, and water is formed when such siloxanes are cured which is unacceptable in ceramic processing (U.S. Pat. No. 2,714,099). Moreover, these polysiloxanes did not contain essentially equimolar amounts of the methylhydrosiloxane and methylvinylsiloxane units. The cyclic siloxane of these references is not useful in ceramic compositions and must be removed.
U.S. Pat. No. 4,578,493 discloses polysiloxanes containing methylhydrosilane and methylvinylsiloxane units in combination with other units, especially dimethylsiloxane units which polysiloxanes are silyl-endblocked. These polysiloxanes are prepared by the cohydrolysis of Me.sub.3 SiCl, MeSiHCl.sub.2, MeSiCl.sub.2 CH.dbd.CH.sub.2, Me.sub.2 SiCl.sub.2 and HSiCl.sub.3 and are used as release coatings at ambient temperatures. The polysiloxanes of this reference contain a large number of dimethylsiloxane units and, thus, are not suitable for ceramic applications.
It is also known that polysiloxanes containing methylhydrosiloxane units are usually equilibrated with an acid catalyst, while polysiloxanes containing methylvinylsiloxane units are usually equilibrated with a base catalyst (The Chemistry and Technology of Silicones, W. Noll, 2nd edition,, (Academic Press: 1968), p.219-229). Further, it does not appear that any single catalyst has been shown to be effective in preparing polysiloxanes containing units of both methylhydrosiloxane and methylvinylsiloxane by equilibration.
U.S. Pat. No. 3,843,702 discloses that trifluoromethanesulfonic acid (CF.sub.3 SO.sub.3 H), commonly referred to as triflic acid, can be a catalyst for the equilibration of lower molecular weight polysiloxanes containing neither silicon to hydrogen bonds nor silicon bonded to a vinyl group. U.S. Pat. Nos. 4,634,610 and 4,634,755 relate to the use of trifluoromethanesulfonic acid to make polysiloxanes having groups such as dimethylsiloxane and methylvinylsiloxane that do not contain silicon to hydrogen bonds. Nowhere do these references disclose an equilibration process for making a polysiloxane containing both silicon bonded to hydrogen and silicon bonded to a vinyl group.
U.S. Pat. No. 4,222,952 discloses that polysiloxanes containing silicon bonded to hydrogen can be rearranged, and, therefore, can be equilibrated, using solid perfluorinated polymers containing pendant sulfonic groups as equilibration catalysts.
U.S. Pat. No. 4,581,391 discloses that methylvinylsiloxane units in polysiloxanes increase the ablation resistance/insulation properties of diorganosiloxane polymers by forming ceramic residue or char on exposure to high temperature. In this reference, the polysiloxanes are cured by the addition of a second polysiloxane containing methylhydrosiloxane units in the presence of a crosslinking platinum catalyst.
U.S. Pat. No. 4,888,376 discloses a hydroxy-endblocked polysiloxane containing methylhydrosiloxane and methylvinylsiloxane units and a siloxy-endblocked polysiloxane having either methylhydrosiloxane units or methylvinylsiloxane units, but not both units in the same polysiloxane. The polysiloxanes of this reference are prepared by hydrolysis.
Accordingly, the prior art does not disclose siloxy-endblocked polysiloxanes containing units of both alkylhydrosiloxane and alkylvinylsiloxane that are effective ceramic precursors for silicon oxycarbide ceramic compositions. Additionally, the prior art does not disclose an equilibration process for preparing siloxy-endblocked polysiloxanes containing units of both alkylhydrosiloxane and alkylvinylsiloxane.